The present invention relates to a system for controlling idle speed of an engine having an idle speed control valve in a bypass around a throttle valve.
The idle speed is controlled by adjusting duty ratio of pulses for driving a solenoid of the idle speed control valve to control air flow passing the bypass. In a conventional idle speed control system (ISC), a driving pulse duty ratio (ISCON) is determined by a basic duty ratio (ISCTW) dependent on the temperature of a coolant of the engine and a feedback correction value (ISCFB) which is a difference between actual idle speed and a desired idle speed dependent on the temperature of the coolant of the engine. Namely, the driving pulse duty ratio ISCON is EQU ISCON=ISCTW+ISCFB
Further, a learning correction value (ISCALT) is added to the duty ratio ISCON in order to stabilize the idle speed.
Japanese Patent Laid Open No. 58-158343 discloses a learning control system for idle speed. In the system, a learning value used for determining the idle speed is stored in a memory when an engine is stopped. When the engine is re-started, the stored learning value is used for the idle speed. At the stop of the engine, the solenoid of the idle speed control valve has a high temperature, because of high temperature of the engine. At high temperature, the solenoid has a larger resistance compared with that of cold engine operating conditions. Accordingly, the duty ratio should be set to a larger value than the cold engine in order to compensate the larger resistance. Therefore, if the learning value stored at the last operation of the engine is used at the re-start of the engine, the value is too large to control the idle speed at cold engine operation, causing increase of engine speed after the engine starts.